Differences in recombination frequencies during female and male meioses ofthe sex chromosomes of the medaka, Oryzias latipes

In the medaka, Oryzias latipes, sex is determined chromosomally. The sex ch romosomes differ from those of mammals in that the X and Y chromosomes are highly homologous. Using backcross panels for linkage analysis, we mapped 2 1 sequence tagged site (STS) markers on the sex chromosomes (linkage group 1). The genetic map of the sex chromosome was established using male and fe male meioses. The genetic length of the sex chromosome was shorter in male than in female meioses. The region where male recombination is suppressed i s the region close to the sex-determining gene y, while female recombinatio n was suppressed in both the telomeric regions. The restriction in recombin ation does not occur uniformly on the sex chromosome, as the genetic map di stances of the markers are not proportional in male and female recombinatio n. Thus, this observation seems to support the hypothesis that the heteroge neous sex chromosomes were derived from suppression of recombination betwee n autosomal chromosomes. In two of the markers, Yc-2 and Casp6, which were expressed sequence-tagged (EST) sites, polymorphisms of both X and Y chromo somes were detected. The alleles of the X and Y chromosomes were also detec ted in O. curvinotus, a species related to the medaka. These markers could be used for genotyping the sex chromosomes in the medaka and other species, and could be used in other studies on sex chromosomes.